Analysis of the low-energy π±p\pi^\pm p differential cross sections of the CHAOS Collaboration

This paper presents the results of an analysis of the low-energy $\pi^\pm p$ differential cross sections, acquired by the CHAOS Collaboration at TRIUMF \cite{chaos,denz}. We first analyse separately the $\pi^+ p$ and the $\pi^- p$ elastic-scattering measurements on the basis of standard low-energy parameterisations of the $s$- and p-wave $K$-matrix elements. After the removal of the outliers, we subject the truncated $\pi^\pm p$ elastic-scattering databases into a common optimisation scheme using the ETH model \cite{glmbg}; the optimisation failed to produce reasonable values for the model parameters. We conclude that the problems we have encountered in the analysis of these data are due to the shape of the angular distributions of their $\pi^+ p$ differential cross sections

Electromagnetic corrections for the analysis of low energy pi-p scattering data

We calculate the electromagnetic corrections to the isospin invariant mixing angle and to the two eigenphases for the s and p-waves for low energy pi-p elastic and charge exchange scattering. These corrections have to be applied to the nuclear quantities obtained from phase shift analyses of the experimental data in order to obtain the hadronic phases. We compare our results with earlier calculations and estimate the uncertainties in the corrections.Comment: 19 pages, 5 figures. Uses elsart.cls Accepted for publication in Nuclear Physics

Pion-nucleon scattering in chiral perturbation theory II: Fourth order calculation

We analyse elastic-pion nucleon scattering to fourth order in heavy baryon chiral perturbation theory, extending the third order study published in Nucl. Phys. A640 (1998) 199. We use various partial wave analyses to pin down the low-energy constants from data in the physical region. The S-wave scattering lengths are consistent with recent determinations from pionic hydrogen and deuterium. We find an improved description of the P-waves. We also discuss the pion-nucleon sigma term and problems related to the prediction of the subthreshold parameters.Comment: 28 pp, LaTeX2e, uses epsf, 6 figs, better discussion of the convergence issues, accepted for publication in Nuclear Physics

Individual based models of avian wildlife populations

An individual-based simulation modeling approach was used in an effort to predict reproductive success of two species of colonially nesting wading birds: the Great Blue Heron (Ardea herodias) and the Wood Stork (Mycteria americana). Two totally different problems were looked upon using the approach. First, the effects of the alteration of foraging conditions for the two species in the Everglades area were considered. The models used for each species followed simultaneously daily activities of individual birds, their spatial movements, foraging efficiency, bioenergetics and growth of the nestlings during a nesting season. For each of the models the effects of reversals were looked upon (that is, increases in depth during the dry season when water levels are normally falling) as a factor affecting reproductive success. Species-specific differences such as visual feeding (for the Great Blue Heron) and tactile feeding (for the Wood Stork were implemented in the models. Simulation results showed that the Wood Storks, frequently foraging in groups, were more affected by the introduced resource scarcity, mainly because of rapid depletion of concentrated resources. Second, the effects of PCB bioaccumulation were studied in a colony of Great Blue Herons. The model considered spatial distributions of PCBs in fish in the colony area. Individual birds were followed and their PCB uptake, bioaccumulation, and retention were modeled. Behavioral responses (foraging deficiency, flight speed) were taking into account using dose-response functions. Simulation results showed that the sublethal behavioral responses in many cases can determine the success of the colony

Pion-nucleon scattering in an effective chiral field theory with explicit spin-3/2 fields

We analyze elastic-pion nucleon scattering to third order in the so-called small scale expansion. It is based on an effective Lagrangian including pions, nucleons and deltas as active degrees of freedom and counting external momenta, the pion mass and the nucleon-delta mass splitting as small parameters. The fermion fields are considered as very heavy. We present results for phase shifts, threshold parameters and the sigma term. We discuss the convergence of the approach. A detailed comparison with results obtained in heavy baryon chiral perturbation theory to third and fourth order is also given.Comment: 44 pp, LaTeX2e, uses epsf, 6 fig

Towards an understanding of isospin violation in pion-nucleon scattering

We investigate isospin breaking in low-energy pion-nucleon scattering in the framework of chiral perturbation theory. This work extends the systematic analysis of [1] to the energy range above threshold. Various relations, which identically vanish in the limit of isospin symmetry, are used to quantify isospin breaking effects. We study the energy dependence of the S- and P-wave projections of these ratios and find dramatic effects in the S-waves of those two relations which are given in terms of isoscalar quantities only. This effect drops rather quickly with growing center-of-mass energy.Comment: 12 pp, REVTeX, 8 figs, FZJ-IKP(TH)-2000-2